Inciting religious revolt/ converting the people to your cause

[Ziaelas]

To quote the manual at this point:

"Assassins never have any STRONG religious beliefs, by the way"

Strong being the key word in that section, they could just be a normal Christian, with light religious beliefs.

[Seven.the.Hun]

well it should be moddable i'd think if any 'pious' ??? assassins are needed.
i agree with furious mental too, been playing a sassanid campaign and the assassin they begin with proves most useful against ERE.

[Patricius]

I have certainly had religious assassins. I would take the words of the manual to believe that there is no chance of them being religious. That Sassanid character - the faction leader, I think - held Colchis for roughly two turns. It had been almost completely converted to Zoroastrianism. His faction collapsed rather quickly and he spent the rest of his life in a fort close to Colchis. I left him alone.